Over the past two decades, there has been a surge of interest in atmospheric pressure plasmas. These include glow discharges, high frequency and dielectric barrier discharges, microwave sustained plasmas, plasma jets and torches, microplasmas, laser-induced plasmas, electron beam generated plasmas, and many others. Typically, their design and operation are tailored for specific applications or to enable different technologies in areas as varied as biology and medicine (see D. B. Graves, “Low temperature plasma biomedicine: A tutorial review,” Phys. of Plasmas 21, 080901 (2014); M. G. Kong, et al., “Plasma medicine: an introductory review,” New J. Phys. 11, 115012 (2009); and X. Lua, et al., “Reactive species in non-equilibrium atmospheric-pressure plasmas: Generation, transport, and biological effects,” Physics Reports 630, 1-84 (2016)); chemistry and material science (see D. Pappas, “Status and potential of atmospheric plasma processing of materials,” J. Vac. Sci. Technol. A 29, 020801 (2011)); aerospace science (see C. L. Enloe, et al., “Surface Potential and Longitudinal Electric Field Measurements in the Aerodynamic Plasma Actuator,” AIAA Journal 46, 2730 (2008)); and environmental engineering (see G. M. Petrov, et al., “Investigation of industrial-scale carbon dioxide reduction using pulsed electron beams,” J. Appl. Phys. 119, 103303 (2016)).
Atmospheric pressure plasmas have certain advantages in materials synthesis and processing that are not available with other approaches including low-pressure plasmas. The breadth of reactions afforded by non-equilibrium, low-temperature plasmas makes them particularly advantageous, and when produced in full density air, such plasmas can be used with systems and materials that are not vacuum-compatible.
One type of non-equilibrium, atmospheric pressure plasmas, often referred to as “plasma jets,” are well-suited for such applications given their relatively simple design, flexible electrode geometry, and modest power requirements. Plasma jets are created when a discharge generated in a confined gas flow, usually a pure or diluted noble gas flowing through a dielectric tube, leaves the region of confinement and propagates through the surrounding ambient. See X. Lu, et al., “Guided ionization waves: Theory and experiments,” Physics Reports 540 123-166 (2016).
FIGS. 1A and 1B illustrate an exemplary conventional apparatus developed at the U.S. Naval Research Laboratory for generating a plasma jet. As shown in the block schematic of FIG. 1A and the photographic image in FIG. 1B, a plasma jet can be generated from a flow 101 of a noble gas such as helium (He), neon (Ne), argon (Ar), krypton (Kr), or xenon (Xe), which passes into a cylinder 102 within an outer casing 106, in which is situated an electrode 103 connected to a voltage source 104. As the gas 101 passes over the electrode 103, it is ionized and forms the plasma jet 105 that is output from the cylinder 102.
Plasma jets can be made quite small, which is good for high-precision applications. See Lua, supra. However, it is difficult to produce jet systems that can scale to treat large surface areas, and as a result, the maximum treatment areas are generally limited to about 1 cm2. See M. Ghasemi, et al., “Interaction of multiple plasma plumes in an atmospheric pressure plasma jet array,” Journal of Physics D: Applied Physics 46, 052001 (2013).
To address the desire for plasma treatment in larger areas, several researchers have constructed one- and two-dimensional arrays of plasma jets, where the treatment area scales with the number of jets. See Ghasemi, supra; see also Q. Y. Nie, et al., “A two-dimensional cold atmospheric plasma jet array for uniform treatment of large-area surfaces for plasma medicine,” New J. Phys. 11 115015 (2009). However, this approach requires increases in power and gas flow, and these increases in power and gas flow also scale with the number of jets. For example, two jets operating in parallel will require twice the gas flow and power input.
It has also been shown that combining two counter-propagating plasma jets effectively extends the length of a plasma discharge. See C. Douat, et al., “Interactions Between Two Counter Propagating Plasma Bullets,” IEEE Trans Plasma Sci. 39, 2298-2299 (2011); see also C. Douat, G. et al., “Dynamics of colliding microplasma jets,” Plasma Sources Sci. Technol. 21, 034010-8 (2012). The two plasma jets can be produced using either two opposing gas flows, each with a corresponding power source and electrode or a single gas stream flowing between two electrodes. In either case, the power requirements effectively double.
Although these approaches clearly work in the sense that the volume and/or effective treatment area increases, the power needed to produce the plasma volume increases as the plasma volume increases. In addition, device complexity increases since both the driving circuit and gas delivery system must be carefully designed so that all of the plasma jets are driven simultaneously and with equal intensity.